This invention pertains generally to the field of scanning thermal probes and particularly to micromachined thermal probes.
A variety of scanning thermal probes have been developed for mapping spatial variations in surface temperatures or the thermal properties of samples. The transducing elements for such devices have included thermocouples, Schottky diodes, bolometer-type resistance change devices, and bimorphs. A bolometer-type sensing element, which maps temperature by fractional changes in electrical resistance, has certain advantage for microcalorimetry applications. In particular, the resistor in the probe can be used to supply heat if sufficient current is passed through it. Because the tip temperature is ultimately influenced by the heat flow between the tip and the sample, variations in thermal conductivity across the sample can be mapped by such a probe. If the heat is supplied by periodic signal, local variations in thermal capacity can also be measured. In essence, because the probe tip serves as a point source of heat as well as a temperature sensor, such devices can be used as a spatially localized microcalorimeter. See A. Hammiche, et al., J. Vac. Sci. Technol. B., Vol. 14, 1996, pp. 1486, et seq.; L. E. Ocola, et al., Appl. Phys. Lett., Vol. 68, 1996, pp. 717, et seq.; D. Fryer, et al., Proc. SPIE, Vol. 333, 1998, pp. 1031, et seq.
Lithography-based micromachining techniques previously used for fabricating scanning probes have generally relied on the removal of the scanning probe from its host substrate or the dissolution of a portion of the substrate in order to provide the necessary clearance for the scanning tip. A fabrication process based on surface micromachining that avoids the need to remove the probe from the substrate is described in M. H. Li, et al., Proceedings of the IEEE International Conference on Micro Electro Mechanical Systems (MEMS ""00), Miyazaki, Japan, January 2000, pp. 763-768. The fabrication process described therein exploits the mechanical flexibility of polyimide to implement an assembly technique that eliminates the need for probe removal or wafer dissolution. An additional benefit of polyimide is that it offers a very high degree of thermal isolationxe2x80x94its thermal conductivity is 0.147 W/mK, in contrast to a thermal conductivity of 141.2 W/mK for silicon. In addition, because this fabrication process has a small thermal budget, it allows the thermal probes to be postprocessed onto integrated circuit chips.
In accordance with the invention, a micromachined scanning thermal probe provides highly sensitive thermal conductance measurements on a wide variety of materials, including heat insulating material such as photoresists and relatively soft material such as biological specimens. The flexibility of the probe allows the tip of the probe to be brought into close proximity to or even into contact with the material being scanned without affecting the material. The scanning probe can also be utilized to carry out topographical measurements by utilizing the probe in a manner similar to an atomic force microscope probe. The probe tip may be heated to progressively higher temperatures while in contact with a sample to allow the detection of the localized glass transition temperature of the sample material at the position of the probe tip.
A micromachined thermal probe in accordance with the invention includes a substrate with a surface and an edge. A flexible probe body includes a cantilever beam section that extends from a proximal end that is secured to the substrate surface outwardly from the edge of the substrate to a distal end. A pair of conductors in the probe body extend to a junction at the distal end of the cantilever beam to allow passage of current through the conductors and through the junction. A probe tip extends away from the cantilever beam at the distal end of the beam in a direction away from the substrate surface and includes a thermally conductive portion which is thermally connected to the conductors at the junction of the conductors. As current is passed through the conductors and through the junction between the conductors, the junction is heated to heat the tip. A change in the thermal conductance of a sample adjacent to or in contact with the tip will change the effective resistance of the conductors and the junction, allowing changes in thermal conductivity as the probe tip is scanned across a sample to be determined. A second flexible probe body may be mounted to the substrate spaced from the first flexible probe body and may be formed to have the same structure as the first flexible probe body. The conductors formed in the second flexible probe body may be utilized as a reference resistance to facilitate compensation of the signal obtained from the conductors in the first probe body to account for ambient conditions such as temperature, etc.
The probe body may be formed of two layers of flexible polymer, such as polyimide, which are joined together over the pair of conductors. The probe bodies may be formed in place on a substrate, such as single crystal silicon, over a sacrificial layer on the substrate surface. After the structure of the probe body is completed, the sacrificial layer may be dissolved to allow release of the probe body from the substrate, with the probe body then being bent back on itself and secured to itself at the proximal end of the cantilever beam, with the cantilever beam then extending outwardly from the edge of the substrate. Layers of gold may be formed on the probe body, which are brought into contact as the probe body is bent over on itself, with the gold layers bonded together by compression bonding to form a strong anchor for the cantilever beam.
The conductors of the thermal probe may be connected in an arm or arms of a Wheatstone bridge circuit to allow current to be provided to the probe conductors and to allow measurement of the resistance of the probe conductors as the probe tip is scanned across a sample. The change in probe resistance may be measured directly to allow determination of changes in thermal conductance, or a feedback circuit may be utilized to supply current to the Wheatstone bridge to maintain a constant temperature at the tip, with the thermal conductance determined from changes in the current signal applied to the probe conductors.
The thermal probes in accordance with the invention are preferably formed utilizing microelectromechanical processing techniques and are preferably formed with dimensions in the range of hundreds of microns or less, e.g., with cantilever beam lengths in the range of 100 to 500 xcexcm, beam widths less than 100 xcexcm, and beam thicknesses of 3 to 10 xcexcm.
Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.